This invention relates to structured packing elements for enhancing contact between fluids.
Various types of exchange columns have been known in which a gas and a liquid (i.e., fluids) come into contact with another, preferably in countercurrent flow. In some cases, use has been made of structured packing elements formed of layers of material (such as plates) that are generally disposed in a plane parallel to the column axis, and that include corrugations which project out of the plane of the layers in order to encourage contact between the liquid and gas. Adjacent layers contact one another at the corrugations. In such cases, the corrugations (which resemble folds in the layers) are disposed at a suitable angle (such as diagonally) to the column axis. Often, the layers also include horizontal fluting to further enhance fluid contact.
In some schemes, each corrugation continues uninterrupted along its entire length, and adjacent corrugations are disposed on opposite sides of the plane of the layer. Thus, the adjacent corrugations respectively resemble diagonal ridges and valleys on the plate. In other arrangements, each corrugation is comprised of a row of corrugations that alternately project oppositely from the plane of the layer so that the corrugations in the row alternate between ridges and valleys along the diagonal. In this latter case, it is seen that flow along a ridge becomes flow along a valley, and vice versa, to increase turbulence and vapor and liquid uniformity along a horizontal cross-section. At the ridge-valley interfaces, liquid flow is displaced from the layer and does not immediately return to the layer.
The corrugated layers are typically made from various materials, for example, textile fabrics (i.e, gauze) stiffened by interwoven metal wires, metallic fabric, fiberglass, plastic, ceramic, or sheet metal. Generally, packing elements which are made of foil-like material, such as sheet metal, are cheaper to produce than packing elements which are made of a self-wetting woven fabric. However, uniform distribution of the liquid (which is important for effective mass transfer or heat exchange with a gas phase) over the surface of a plate of foil-like material may not occur, because capillary forces may not come into operation.
Similarly, vapor flow up through the corrugations is sensitive to pressure differentials, generated in part by non-turbulent flow, pressure drag due to the generation of normal stresses in the flow, and non-homogeneous flow patterns through the grid structure. In sections of the flow up through a corrugation, flow tends to be more laminar going up the valley of a corrugation and turbulent where it is crossing a corrugation. Frictional drag along all channels and all adjacent openings provides uniformity and less propensity for fluid displacement and maldistribution or channeling of either the vapor or the liquid through certain portions of the packing element grid or its layers. Such dynamic action directly affects efficiency and mass heat interaction.
In some packing elements, the absence of any substantial degree of uniformly imparted frictional drag, generated by tangential stresses, through the adjacent corrugations and fluted areas decreases the uniformity and homogeneity of the flow pattern throughout the grid and reduces the efficiency and mass heat transfer characteristics of the packing.